1. Technical Field
The present invention relates to a hydrodynamic bearing having fluid in its rotation section and a disc rotation apparatus having the same.
2. Prior Art
In recent years, in recording apparatuses using discs and the like, their memory capacities are increasing and their data transfer speeds are rising. Hence, a disc rotation apparatus for use in this kind of recording apparatus is required to rotate at high speed and with high accuracy, and a hydrodynamic bearing is used in its rotating main shaft section.
A conventional hydrodynamic bearing and an example of a disc rotation apparatus having the same will be described below referring to FIG. 12. FIG. 12 is a cross-sectional view showing the right portion of the center of the rotation shaft, that is, the center line C, of the conventional hydrodynamic bearing. In FIG. 12, a shaft 31 is rotatably inserted into a sleeve 32 having a bearing hole 32A. At the lower end of the shaft 31, a flange 33 is provided so as to be integrated therewith. The lower end of the flange 33 is accommodated in a recess portion formed by a hole in a base 35 and the sleeve 32 and rotatably held so as to be opposed to a thrust plate 34 mounted on the base 35. A hub rotor 36, a rotor magnet 38, a plurality of discs 39, spacers 40 and a clamper 41 are secured to the shaft 31. A motor stator 37 opposed to the rotor magnet 38 is installed on the base 35. Dynamic pressure generation grooves 32B and 32C indicated by broken lines are provided on the inner circumferential face of the bearing hole 32A of the sleeve 32. Dynamic pressure generation grooves 33A are provided on the upper face of the flange 33, a face opposed to the sleeve 32. In addition, dynamic pressure generation grooves 33B are provided on the lower face of the flange 33, a face opposed to the thrust plate 34. The clearances between the shaft 31 and the sleeve 32, including the dynamic pressure generation grooves 32B, 32C, 33A and 33B, are filled with oil.
The operation of the conventional hydrodynamic bearing shown in FIG. 12 will be described below. In FIG. 12, when electric power is applied to the coil of the stator 37, a rotating magnet field is generated, and a rotation force is generated in the rotor magnet 38, whereby the shaft 31 and the flange 33 rotate together with the hub rotor 36 and the discs 39. During the rotation, dynamic pressures are generated in the oil by the dynamic pressure generation grooves 32B, 32C, 33A and 33B, and the shaft 31 is floated in the upward direction of the figure and rotates while holding space from the sleeve 32 and without making contact with the thrust plate 34 and the sleeve 32. Magnet heads, not shown, make contact with the discs 39 and carry out the recording and reproduction of electrical signals.
The conventional hydrodynamic bearing configured as described above had problems described below. FIG. 13 is a plan view of the flange 33 which is provided with a plurality of the dynamic pressure generation grooves 33A indicated by black-colored regions. FIG. 14 is a bottom view of the flange 33 which is similarly provided with a plurality of the dynamic pressure generation grooves 33B indicated by black-colored regions. The outside diameters of the patterns of the dynamic pressure generation grooves 33A and 33B on the top and bottom faces are represented by D1o and D2o, respectively, and their inside diameters are represented by D1i and D2i, respectively. The diameters D1m and D2m of the respective turn-back parts of the dynamic pressure generation grooves 33A and 33B are set at sufficiently large values so that pumping pressures in the directions indicated by arrows E and F and by arrows G and H, respectively, are raised.
FIG. 15 and FIG. 16 are views showing the cross sections of relevant parts in the vicinity of the lower end of the shaft 31 and showing pressures on the surfaces of the flange 33 and the shaft 31 of the above-mentioned conventional hydrodynamic bearing. If the pumping pressures in the directions indicated by arrows E and H shown in FIG. 13 and FIG. 14, respectively, are raised too high, a negative pressure with respect to atmospheric pressure is generated at the central portion of the lower face of the flange 33 as indicated by curve P1 in FIG. 15, whereby air bubbles mixed in the oil are coagulated and air is accumulated in a region 43B having a constant size.
In FIG. 16, the dynamic pressure generation grooves 32B and 32C of the sleeve 32 are made so that dimension L1 in the figure is larger than dimension L2, (L1>L2), and so that dimension L4 is larger than dimension L3, (L4>L3). In addition, the dimensional difference (L1−L2) is selected so as to be nearly equal to the dimensional difference (L4−L3), that is, (L1−L2)≈(L4−L3). As shown by ΔL in FIG. 16, in the case that the amount of the oil becomes slightly insufficient and the upper face of the oil is at the position lower than the upper ends of the dynamic pressure generation grooves 33B by dimension 4L, no oil is present in the portion corresponding to the dimension ΔL of the upper ends of the dynamic pressure generation grooves 33B, whereby the pressure distribution of oil is represented by curve P2 shown in FIG. 16. In addition, a negative pressure is generated at the lower portion of the range of the dimension L4 in the figure. Hence, air bubbles are accumulated in a region 43A, whereby there is a fear of breaking the oil film in this region 43A and of causing friction between the shaft 31 and the sleeve 32.